1. Field of the Invention
The present invention relates to an exhaust purification device for an internal combustion engine.
2. Description of Related Art
Generally, in a diesel engine for a vehicle or the like, combustion (lean combustion) is performed in a state of a lean air fuel ratio (air fuel ratio corresponding to fuel ratio lower than that of stoichiometric air fuel ratio). Accordingly, a quantity of NOx (nitrogen oxides) contained in exhaust gas increases. Therefore, reduction of a NOx emission quantity is required from the viewpoint of environmental protection.
In recent years, a device using a NOx occlusion reduction catalyst, i.e., a NOx catalyst, has been gathering attention as one of exhaust purification devices for purifying the NOx contained in the exhaust gas. The NOx catalyst used for the device consists of an alkaline earth material (occlusion material) and platinum, for example. The NOx catalyst has characteristics of occluding the NOx in the exhaust gas when the atmosphere of the exhaust gas is the lean air fuel ratio and of reducing and eliminating the occluded NOx with the use of reduction components such as HC and CO contained in the exhaust gas when the air fuel ratio becomes rich (air fuel ratio corresponding to fuel ratio higher than that of stoichiometric air fuel ratio). This device uses such the characteristic of the catalyst. The device repeats the occlusion and the reduction (discharge) of the NOx with the catalyst to purify the NOx in the exhaust gas and to reduce the NOx emission quantity.
However, also in such the device, there is limitation in the occlusion capacity of the NOx catalyst. Therefore, if the catalyst is continuously used in the environment where the NOx reduction quantity (NOx discharge quantity) exceeds the NOx occlusion quantity and the NOx occlusion quantity approximates to the occlusion limit, the NOx purification capacity of the catalyst declines significantly. Therefore, conventionally, a known exhaust emission purification device periodically reduces and removes the NOx occluded by the NOx catalyst as processing (catalyst recovery processing) for recovering from the fall of the NOx purification capacity (temporary performance degradation corresponding to NOx occlusion quantity) (for example, as described in JP-A-2000-34946 or Japanese Patent Gazette No. 2692380).
Next, with reference to FIGS. 9A and 9B, an outline of the NOx catalyst recovery processing currently performed by conventional and general devices including the above-described device will be explained. As an example, an exhaust gas purification system for a vehicular diesel engine performing usual operation by the lean combustion will be explained. The lean combustion is performed in the engine as the object of the exhaust gas purification. Therefore, after the NOx occlusion with the NOx catalyst is started, the NOx occlusion quantity Qnox increases with time as shown by a solid line L5a in FIG. 9A. The NOx purifying performance (calculated as NOx purifying rate Rnox in this example) of the NOx catalyst decreases as shown by a solid line L5b in FIG. 9B as the NOx occlusion quantity Qnox increases.
The device serially performs recovery processing to discharge substantially an entirety of the NOx occluded in the NOx catalyst (entirety or large quantity close to entirety) once every time an execution condition for the processing is satisfied (for example, at each elapse of a predetermined time). Specifically, the device performs oversupply of the fuel (i.e., rich purge), for example, at timing t50 at which the execution condition is satisfied, to make the air fuel ratio in the NOx catalyst rich temporarily. Thus, the occluded NOx is reduced by the HC, the CO and the like contained in the exhaust gas and removed. The device periodically performs the rich purge to reduce and remove substantially the entirety of the occluded NOx to periodically recover the purifying performance (exhaust purification performance) of the NOx catalyst.
In such a device, by appropriately setting the execution condition (for example, by setting an appropriate execution interval), the recovery processing (rich purge) is performed before the purifying rate falls below a required value TR (required purifying rate). The purifying capacity of the NOx catalyst is recovered every time the recovery processing is performed. As a result, the NOx catalyst can be used continuously. Thus, each of the conventional devices including the above-described device provides the system (i.e., LNT system: Lean NOx Trap system) that reduces the NOx by making the air fuel ratio rich temporarily through oversupply of the fuel. The system repeatedly performs the recovery processing as shown in FIGS. 9A and 9B based on the appropriate execution condition to continuously purify the exhaust gas (NOx) to thereafter operate at a high purifying rate.
However, the exhaust gas purification performance of the NOx catalyst does not necessarily depend only on the NOx occlusion quantity. Rather, in some cases, performance degradation of the catalyst can be caused by other factors than the NOx occlusion quantity. For example, since a sulfur component is contained in the engine fuel, SOx (sulfur oxides) as oxides of sulfur are generated during combustion and is also occluded by the NOx catalyst like the NOx. Since SOx is a chemically stable substance, it is difficult to release SOx from the NOx catalyst even by making the air fuel ratio rich. Therefore, if a state of increased SOx occlusion quantity (sulfur poisoning) is reached, the NOx purifying capacity will decline by SOx occlusion quantity. In the state where sulfur poisoning has arisen, in a graph (time chart) showing the purification characteristic of the NOx catalyst at the time of applying the recovery processing shown in FIGS. 9A and 9B to the NOx catalyst, the NOx purifying rate falls as a whole, for example, as shown by broken lines L6a, L6b in FIGS. 9A and 9B respectively, compared to the graph shown by the solid lines L5a and L5b. Therefore, as shown in FIG. 9B, there is a concern that the purifying rate Rnox of the NOx catalyst becomes less than the demanded purifying rate TR, for example, at timing t51 before the timing t50 at which the recovery processing is applied to the catalyst.
That is, with the above-described conventional device, even if the execution condition is once set appropriately, the adequacy of the condition will be lost if degradation (for example, sulfur poisoning) arises in the NOx catalyst in connection with the elapse of time. Accordingly, there occurs a concern that the purifying rate (exhaust gas purification performance) of the catalyst falls below the demanded purifying rate.